July 23, 2015

Joist vs. Joist Girders in RISAFloor

RISAFloor has the ability to design standard steel joist.  This includes not only joists but joist girders as well.  Here we will explore the steps to model a joist vs a joist girder.


Allowable loads for Steel Joists are based on the standard load and span tables for K, LH, DLH, and SLH Steel Joists.  When drawing a joist in the model, the Steel Product option should be selected in the Draw Beams dialog box.  Here you can use the Shape Group dropdown box to select the joist group:

Joists 1

Another option to specify the steel product is in the Beams spreadsheet under the Shape column.  A similar dropdown box will available to select your joist group.

Joists 2

Once a joist group is selected, the program will automatically optimize the member based on the minimum weight and based on the criteria specified in the Design Rules spreadsheet.  This also includes the joists ability to resist the UDL, ULL, and UTL.

You also have the ability in the Beams spreadsheet to define an explicit shape by selecting this option in the Set Member Shape dialog box and selecting Explicit Shape and Shapes List.  This will bring up the Shape Selection dialog box to specify a joist size.

Joists 3

“Generic Steel Joist” is an option in the dropdown boxes which will automatically optimize the member based on minimum weight and based on the criteria specified in the Design Rules spreadsheet.

Joist Girders

The program will perform Joist Girder design according to the basic Joist Girder tables given in the Steel Joist Institute tables and/or the Vulcraft catalog.  The intended use is that you will designate the depth of the Joist Girder before the model is solved.  If a generic “Joist Girder” is chosen, then the program will optimize the beam by finding the Girder with the least depth. This will often be a heavier member than is necessary for design.  For this reason it is strongly suggested that you provide a Girder depth whenever you specify a joist Girder.

RISAFloor will call out a Joist Girder in the design results using the following terminology:



  • 16G refers to the DEPTH of the Joist Girder (16 inches for this example)
  • 4N corresponds to the NUMBER of equally spaced concentrated loads (4 for this example)
  • 4K refers to the MAGNITUDE of the concentrated loads (2.4 kips for this example)

Joist girder depth can be selected in the Draw Beams dialog box.  The program will then optimize the member within this depth group:

Joists 4

In the Beams spreadsheet, a generic “Joist Girder” can be selected under the Design Group option and using the Shape Selection dialog box:

Joists 5

Alternatively, the specific girder size can be specified under the Explicit Shape option:

Joists 6


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